1. Field of the Invention
This invention relates to toll restricting telephone apparatus and, more particularly, to an improved attachment apparatus for preventing the placement of long distance telephone calls from a restricted telephone set.
2. Description of the Prior Art
It has heretofore been known to provide toll restriction circuits on various telephones in a telephone system so as to prevent the use of those telephones for long distance telephone calls for obvious reasons of economy. In the prior art system of which I have knowledge, restrictive equipment was maintained at the central station which received the full information on the called number, determined whether the called number was a long distance call, and determined whether the telephone station from which the call was placed is authorized to make such long distance calls and when that determination was negative the line was disabled and the call was not completed.
Currently many users install private automatic branch exchanges (PABX) which they interconnect to telephone company lines. Ideally it is desired to restrict some of the telephone stations within the PABX system from initiating off-premises long distance telephone calls without disturbing the other substations therein or without requiring the installation of expensive circuitry of the aforementioned type frequently found in central stations.
Toll restricting devices for the foregoing application have been made known to me in U.S. Pat. No. 3,727,007 to Catlin, granted Apr. 10, 1973, and in U.S. Pat. No. 3,757,055 to McCann, granted Sept. 4, 1973. In each of the toll call restrictors disclosed in the aforecited patents, means are provided to terminate the placement of the telephone call if the number of digits dialed exceeds that necessary for a local call or if the initial digit or digits dialed represents an attempt to reach a long distance telephone operator. The present invention is directed to a similar type of toll restricting apparatus.
In my prior application, application Ser. No. 558,302, filed Mar. 14, 1975, referred to and incorporated herein in its entirety, I present what I believe to be a novel apparatus and method for preventing placement of long distance telephone calls from a restricted local station by counting the number of digits dialed at that station and once the count attains a predetermined number, such as the predetermined number of digits representative of a local call, by preventing the sending of the further digits necessary to complete the identification of the long distance station. In the embodiment disclosed in that application, I employ as part of the elements thereof a shift register, a pair of Nand gate latches and various trigger circuits associated therewith. In accomplishing the aforedescribed purpose and to prevent dialing of a zero as the initial digit, the electronic shift register requires the insertion of an initial information bit.
Additionally inverters were required to couple the output from the counter to the respective Nand gate latches. Such a circuit I found to be more complicated and thus less reliable than desired.